The invention relates to microwave mixers which consist of active nonlinear electronic elements of the type comprising diodes or transistors and which are more particularly applicable to the field of telecommunication receivers.
The term mixer is understood to mean an electronic device for mixing two periodic signals having different frequencies in order to deliver at its output a single signal, the frequency spectrum of which contains at least one frequency equal to the sum or to the difference of the frequencies of the two signals applied to the mixer, the difference frequency being commonly referred-to as an intermediate frequency.
A basic problem to be solved when it is sought to construct a mixer lies in the need to achieve maximum reduction of conversion losses. Conversion losses correspond to the difference, usually measured in decibels, between the power available at the input of the mixer and the power available at its output.
In theory, conversion losses could be zero if the output were perfectly adapted to the frequencies of the spectral lines of the output signal of the mixer and if all the powers of the nonessential lines of the spectrum were recovered.
In practice, however, it is impossible to control all the terminal impedances of a mixer in respect of each frequency. In consequence, conversion losses are never zero and the higher the losses, the more they produce a harmful effect on the sensitivity of the receiver which is coupled to the output.
In order to reduce conversion losses, one known method consists in placing on the signal access of the mixer band-stop filters tuned to predetermined frequencies of the spectrum of the output signal in order to reflect from the nonlinear electronic elements the corresponding lines of the mixed output signal and in order to obtain by further mixing with other preferential frequencies of the spectrum signal components in phase with and having the same frequency as the intermediate-frequency signal F.sub.O obtained by direct mixing.
By way of example, it is already a known practice to fabricate mixers of the band-stop filter type in the form of microcircuits. These mixers are formed by waveguides or slotted lines deposited by metallization on a substrate for recovering and reflecting the image frequency F.sub.I at the level of the nonlinear electronic elements of the mixer. This image frequency F.sub.I is symmetrical with the frequency F.sub.S of the signal applied to one input of the mixer with respect to the frequency of the mixing signal F.sub.L applied to the other input. The image frequency F.sub.I =2F.sub.L -F.sub.S produced by the nonlinear electronic elements is reflected from these latter by the band-stop filter and further mixing of the image frequency F.sub.I with the frequency of the mixing signal produces a frequency F.sub.L -F.sub.I =F.sub.L -(2F.sub.L -F.sub.S)=F.sub.S -F.sub.L =F.sub.O which is the same as the intermediate frequency obtained by direct mixing of the frequencies, namely the incident frequency F.sub.S and the mixing frequency F.sub.L. Provided that the corresponding signal obtained is in phase with the intermediate-frequency signal obtained by direct mixing, these devices make it possible to obtain an increase in output amplitude of the intermediate-frequency signal F.sub.O and consequently a reduction in conversion losses in accordance with the desired objective.
Similarly, another expedient which could be considered with a view to reducing conversion losses would consist in reflecting the "sum" frequency resulting from direct addition by the mixer of the input frequencies (F.sub.L +F.sub.S) towards the nonlinear electronic elements, this being also achieved by means of a band-stop filter tuned to the "sum" frequency. Mixing of the frequency F.sub.L +F.sub.S with the double frequency 2F.sub.L of the mixing signal also results, in this case, in an intermediate frequency F.sub.O in accordance with the operation: EQU F.sub.O =(F.sub.L +F.sub.S)-2F.sub.L =F.sub.S -F.sub.L
On condition that the signal having a frequency F.sub.O and obtained as a result of the operation stated above is in phase with the intermediate-frequency signal, an additional reduction in conversion losses is necessarily obtained.
Unfortunately, in practice, control of the "sum" frequency F.sub.S +F.sub.L cannot be obtained by means of techniques for the fabrication of waveguides in a microcircuit by reason of the various parasitic effects arising from the appearance of surface waves, from dispersion of the microwave-strip or slotted-line waveguides employed and from resonance effects produced by the casing and cover containing the electronic circuits of the mixer. These difficulties arise especially from the fact that the "sum" frequency is located at approximately double the frequency of the signal applied to the input of the mixer since, as a rule, the intermediate-frequency signal has a frequency F.sub.O which is well below the frequency F.sub.S of the signal applied to the input of the mixer.